Currently, a rechargeable battery can be charged in one of the following four modes, namely, constant current (CC) charging mode, constant voltage (CV) charging mode, pulse charging mode, and constant current-constant voltage (CC-CV) charging mode.
In the CC charging mode, the battery can be fully charged within a short time. However the charging efficiency becomes poor, when the battery is close to fully-charged state. Besides, an overcharge protective mechanism is required to protect the battery from overcharge. Therefore, the CC mode charging circuit requires a high cost.
In the CV charging mode, the charging circuit regulates the magnitude of charging current according to a voltage difference between the battery voltage and a pre-defined charging voltage. That is, the charging current gradually drops when voltage difference becomes smaller. Since the CV charging mode does not require a protective mechanism to protect the battery against overcharge, the relatively larger voltage difference at the early stage of charging would possibly result in an overcharged state to cause damage to the battery.
In the pulse charging mode, a pulse current is periodically supplied to charge a battery. That is, in the process of charging, the electrolyte in the battery have plenty of intervals to buffer neutralization in the chemical reaction such that different electrolytes can balance chemical substance drift to improve the uniformity of electrolyte reaction and accordingly, the life of the battery. However, up to date, there is not a recognized way for determining the best pulse charging frequency and interval.
In the CC-CV charging mode, the battery is initially charged in the CC mode. When the battery voltage has reached at a pre-defined voltage, the battery is then charged in the CV mode such that the charging current drops with the rise of the battery voltage. When the charging current drops to zero, it is considered that the battery has been 100% charged. However, in the CC-CV mode, since it must wait until the charging current drops to zero in the final stage of charging, the charging time is prolonged.
Therefore, all the prior art charging circuits fail to enable fast charge and overcharge protection at the same time.